| Coral reef ecosystems distributed in tropical oceans are extremely sensitive to environmental changes,and the calcium carbonate（coral reef rocks）secreted by corals and other reef-building organisms are also sensitive to respond to and record the environmental changes,including the rise and fall of sea level and changes in seawater chemical characteristics.However,due to the limitations of sample acquisition,diagenesis and other factors,there are few studies on reef rock geochemistry in recording sea-level changes and palaeo-environmental changes over a long time series.This research studied the coral reef deep drilling-Chenke 2[CK2,928.75 m,including the upper 878.2 m for coral reef carbonate,the lower 50.52 meters for the volcanic rocks,the bottom of coral reef rocks is 19.6 Ma（one million years）]in the Chenhang island,Yongle Atoll,Xisha islands in the Northern South China Sea.Based on the detailed observation and description of the core,samples were selected for strontium isotope,carbon and oxygen isotope,and element testing and analysis,in order to explore the geochemical characteristics of coral reef carbonate and its environmental significance in the Xisha Islands during the past 20 Ma.This study focuses on the following scientific questions:1)the variation of the main biological components and features of exposed surface of the coral reef carbonate in Xisha Islands during the past 20 Ma;2)the element,strontium isotopic and the carbon and oxygen isotopic composition of coral reef carbonate and their environmental significance of the coral reef carbonate in Xisha Islands during the past 20 Ma;3)Sea level fluctuation and tectonic activity which record by the coral reef carbonate in Xisha area during the past 20 Ma.The main conclusions of this study are as follows:1)The biotic components and petrology features of coral reef carbonate can record the relative sea level change.The main biotic components of coral reef carbonate in CK2 is corals,coralline algae,Halimeda,foraminifera,bivalves and gastropods.And different organisms were dominant in different periods,indicating the environmental changes in which coral reef development.Corals were the main reef-building organisms at 0-190 m and 434.66-878.22 m,indicating relatively shallow water depths at these two stages.However,rhodolith and coralline algae were the main reef-building organisms at the depth of 190-434.66 m,indicating a relatively deep water environment at this stage.In addition,In addition,there are also 31 layers which dominated by Halimeda,8layers which dominated by foraminifera,and 2 layers which dominated by seashell were identified in CK2,implying the environmental fluctuation in Xisha area during the past 20 Ma.Coral reefs are formed in sea water,and their exposed surfaces are formed by weathering and leaching caused by sea level decline.Compared with the white coral reef carbonate formed under normal environment,the coral reef carbonate at and near the exposed surface are usually yellow,brownish yellow and maroon red.A large number of voids and dissolution pores can be seen in the core.The surface of the pores is mostly covered with calcareous crusts and clayey mud materials,and some of the pores develop stalactite-shaped crystals.Elev exposed surfaces were identified in CK2,indicates that there have been at least eleven relatively low sea level periods in Xisha Islands during the past 20 Ma.2)The element content of coral reef carbonate are impressionable by the chemical composition of seawater and diagenesis,which can indicate the environmental changes in the formation process.They can be divided into 4types based on the similar changes:（1）The element content are influenced by dolomitization.Mg,Mn,P,Ba,Cr,Y,Cd and Pb are enriched,while Ca and Sr are lost in the process of dolomitization.And dolomite content is positively correlated with Mg content and negatively correlated with Ca content,indicating that the changes of Mg and Ca content in coral reef carbonate can reflect the strength of dolomitization to a certain extent.（2）The element content are sensitive to the meteoric water.The contents of Mn,Ti,Zr,Al,Si,K and Fe are high,while the content of Sr is low near the exposed surface of CK2,indicating that they are susceptible to atmospheric water alteration,and can record sea level fall.（3）The element can records the chemical composition of seawater.The coral reef carbonate REE patterns of CK2 are consistent with seawater in South China Sea,showing low LREE,high HREE,low Ce,and high Y patterns,indicating that the coral reef carbonate REE originated from seawater,and can record the changes of seawater REE.（4）The element content are stable relatively.The overall trend of Co,Cu,Zn,V and Ni content do not change significantly,indicating that they are not affected by dolomitization and atmospheric water alteration.In addition,the results of the content of Mn,Sr content and Mn/Sr,Sr/Ca,which are commonly used to determine the degree of carbonate alteration,show that most of the CK2 coral reef carbonate have very low degree of alteration,except samples at 672-616,596-575,521-491 and192.5-161 m which have been altered to some extent.3)The 87Sr/86Sr of coral reef carbonate is controlled by the chemical composition of seawater,diagenesis and the development of coral reefs,which not only has chronological significance,but also record the sea level changes during their formation.The 87Sr/86Sr of coral reef carbonate in CK2 showed a continuously increasing trend with shallower depth,from 0.708509±10 at the bottom to 0.709164±8 at the top.The overall variation range and trend were consistent with the variation range and trend of 87Sr/86Sr of seawater since 19.6Ma,showing that the 87Sr/86Sr of the South China Sea and the open ocean are homogeneous in the last 20 Ma,and can be used to reconstruct the chronological framework based on Strontium Isotopic Stratigraphy.Compared with seawater87Sr/86Sr,the coral reef carbonate 87Sr/86Sr of CK2 showed three types of changes:（1）The 87Sr/86Sr increased continuously and monotonically,which reflected that the 87Sr/86Sr of coral reef rocks was mainly affected by seawater.Therefore,it could be used to reconstruct the chronological framework,and indicate the continuous development of coral reefs vertically and the relative sea level rise.（2）The 87Sr/86Sr is higher than the upper and lower level,which usually appeared near exposed surfaces along the core,indicating that the sea level is falling and the reef rocks are exposed and altered by atmospheric precipitation.（3）A sudden jump increase in 87Sr/86Sr,and result in a break in the continuous rising 87Sr/86Sr curve.Which usually appeared near exposed surfaces,indicating that the relative sea level is falling or stable and the development of coral reefs is limited in vertical direction.4)Theδ13C andδ18O of coral reef carbonate are affected by the chemical composition of seawater,diagenesis,mineral composition and environment.And theδ13C can reflect theδ13C of dissolved inorganic carbon（DIC）in surface seawater,theδ18O can indicate the temperature change during its formation.The coral reef carbonateδ13C andδ18O of CK2 varied widely,ranged-6.27-（19）（13）（18）（22）‰（average 0.47‰）and-8.75-（19）（13）（20）（19）‰（average 1.99‰）,respectively.The dolomiteδ13C andδ18O are higher,ranges 0.41-（17）（13）（24）（18）‰and 0.55-（19）（13）（20）（19）‰,respectively.In addition,theδ13C andδ18O near the exposed surface showed a negative shift,indicating the coral reef carbonateδ13C andδ18O are sensitive to the meteoric water,and would be lower with the reaction of meteoric water.We selected the single mineral-dolomiteδ13C andδ18O to study their controlling factors and environmental significance,due to the fact that the coral reef carbonateδ13C andδ18O are greatly affected by mineral composition.The microstructural and geochemical features suggest that the dolomite of CK2 was transformed from the initial coral reef carbonate in the seawater environment during 10-0.5 Ma.The dolomite inherited the precursor mineralsδ13C during the dolomitization,thus can record the change of dissolved inorganic carbonδ13C of surface seawater.δ18O is easily affected by many factors,but is most closely related to temperature.（5）Coral reef rocks can record past sea level changes and regional tectonic activity.We reconstructed the relative sea level changes in the northern South China Sea based on the geochemical characteristics,exposed surface and the depth of weathering leaching over the past 20 Ma,including 14 times of sea level rise and 13 times of sea level fall.Among the 13 times sea level fall,eight were recorded by both exposed surface and 87Sr/86Sr,two were recorded by the exposed surface but not 87Sr/86Sr,three were recorded by 87Sr/86Sr but not at the exposed surface.The relative sea level rise is slow and long-term,occurred during 19.6-18.6、18.3-17.4、17.2-16、16-15.4、15.4-12.4、10.2-9.6、9.6-7.4、7.4-5.8、5.2-4.5、4.0-1.8、1.8-1.1、0.9-0.3、0.3-0.2 and 0.2-0 Ma,mainly controlled by the tectonic subsidence of the Xisha block and global sea level.The relative sea level fall in the northern South China Sea was mainly controlled by the global sea level fall.The sea level fall during 18.6-18.3、17.4-17.2、12.4-10.2、5.8-5.2、4.5-4 and 1.1-0.9 Ma was long-term and slow,which was mainly driven by global cooling and the expansion of continental ice sheet.The sea level fall at 16、15.4、9.6、7.4、2.2、0.3 and 0.2 Ma are short-term and rapid,and the driving mechanism is unclear.Based on the relative sea level change history of the northern South China Sea and the global sea level change database,we estimated that the tectonic actively of Xisha block are dominated by subsidence in the past 20 million years,the subsidence rates are lowest with13.3 m/Ma during 19.6 Ma-18.6 Ma;decreased from 105 m/Ma to 24.7 m/Ma during 18.3-7.4 Ma;fluctuated during 7.4-2.2 Ma,increased from 35.3 m/Ma to 83.6 m/Ma,and down to 50 m/Ma again;highest with 172.9 m/Ma during2.2-1.1Ma;down to 78.1 m/Ma during 0.9-0.2 Ma again.The coral reef carbonate of CK2 record 13 times of global sea level fall,the age（and magnitudes）of the global sea level fall are 18.6-18.3 Ma（18-28 m）,17.4-17.2Ma（10 m）,16 Ma（69.04-79.04 m）,15.4 Ma（5 m）,12.4-10.2 Ma（69.2-79.2m）,9.6 Ma（15-20 m）,7.4 Ma（54-59 m）,5.8-5.2 Ma（26 m）,4.5-4 Ma（29.79m）,2.2 Ma（21.7-26.7 m）,1.1-0.9 Ma（48 m）,0.3 Ma（17 m）and 0.2 Ma（21.96 m）,respectively.These interpreted sea level falls are mostly consistent with other records of global sea level changes,confirming the reliability of our records.Collectively,these results indicate that coral reef carbonate can record the87Sr/86Sr and REE of seawater,and the dissolved inorganic carbonδ13C of surface seawater during the geologic period.In addition,coral reef carbonate are good carrier to record sea level changes.During sea level fall,coral reef carbonate not only show obvious rock characteristics anomalies,but also obvious geochemical anomalies.Based on this,we can established the history of sea level changes and regional tectonic activities.Based on the comprehensive analysis of geochemical feature and controlling factors of coral reef carbonate in CK2,this study suggests that coral reef carbonate can not only be used as reliable carriers to record the seawater geochemical signal,but also can record and respond to the environmental changes experienced sensitively,which has great potential in the study of climate and environmental changes in geological period. |
